A variety of catalyst compositions containing single site catalyst precursors have been shown to be highly useful in the preparation of polyolefins, producing relatively homogeneous copolymers at good polymerization rates and allowing one to tailor the properties of the finished polymer closely. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions comprise catalytic compounds in which each catalyst composition molecule contains one or only a few polymerization sites.
The most well known category of single site catalyst precursors is metallocenes of the general formula Cp.sub.2 MX.sub.2 wherein Cp is a cycloalkadienyl ligand, typically cyclopentadienyl or indenyl, M is a metal, usually from Group 4, and X is a halogen or alkyl group.
Other types of single site catalyst precursors have more recently been reported. Wolczanski et al., Organometallics, 1:793 (1982) describes the synthesis of Cp*Zr(BH.sub.4).sub.3 and a related dimer, [Cp*Zr(BH.sub.4)H(.mu.-H)].sub.2, wherein Cp* is pentamethylcyclopentadienyl. The authors state on page 794, the dimer "appears to polymerize ethylene; however, the low rate of oligomerization may indicate trace impurities are responsible."
The present invention revolves around the discovery that single site catalyst precursors comprising at least one protected hydride or protected hydrocarbyl ligand bound to a metal atom combined with a cocatalyst are particularly effective for the polymerization of olefins. Protected hydride or hydrocarbyl ligands are quite stable when attached to the metal of ligated catalyst precursor. In contrast to the findings of Wolczanski et al., this unique combination of precursor and cocatalyst provides an extremely active catalyst composition.